The purpose of this study is to elucidate the mechanisms of water movement across the cochlear partition. Systemic injection of glycerin or deionized water was used to induce alterations in the osmolarity of the blood serum in guinea pigs. The cochlear potential including the endocochlear potential did not show marked suppression when the osmolarity of the cochlear fluids was elevated or lowered. Under normal conditions the osmolarity is significantly higher in the endolymph than perilymph. When the blood osmolarity was altered, the osmotic gradient across the cochlear partition decreased. These studies are used as base line data to which water control in the cochlear fluids under pathologic conditions can be compared.